Among water-soluble polymers such as polyacrylic acid and polymaleic acid, low molecular weight polymers have been suitably used in agents such as dispersants for inorganic pigments and metal ions, for example, and water treatment agents. For example, Patent Literature 1 discloses a (meth)acrylic acid-based copolymer containing a structural unit (a) derived from a (meth)acrylic acid-based monomer (A) having a specific structure, and a structural unit (b) derived from a monoethylenically unsaturated monomer (B) copolymerizable with the (meth)acrylic acid-based monomer (A), wherein a sulfonic acid group is present at least one end of the main chain (the sulfonic acid group may be an ammonium salt, alkali metal salt, or salt of an organic amine group); the structural unit (b) contains at least a structural unit (b1) derived from a (meth)allyl ether-based monomer (B1) having a specific structure; and the ratio between the structural unit (a) and the structural unit (b1) is as follows: 70 to 95% by mole of the structural unit (a) and 5 to 30% by mole of the structural unit (b1).
Patent Literature 2 discloses a (meth)acrylic acid-based copolymer containing, as essential structural units, a structural unit (a) derived from a monomer having a specific structure in an amount of 5% by mole or more and 22% by mole or less and a structural unit (b) derived from a (meth)acrylic acid (salt) in an amount of 78% by mole or more and 95% by mole or less, relative to 100% by mole of all structural units derived from all monomers, wherein a sulfonic acid (salt) group is present at least one end of the main chain, and the weight average molecular weight is 13000 to 50000.
Patent Literature 3 discloses a (meth)acrylic acid-based copolymer containing, as essential structural units, a structural unit (a) derived from a monomer having a specific structure in an amount of 5% by mole or more and 25% by mole or less, a structural unit (b) derived from a (meth)acrylic acid (salt) in an amount of 50% by mole or more and 88% by mole or less, and a structural unit (c) derived from maleic acid (salt) in an amount of 7% by mole or more and 25% by mole or less, relative to 100% by mole of structural units derived from all monomers, wherein the weight average molecular weight is 1000 to 30000.